Transistors are critical components for implementing digital and analog circuit designs. Generally a MOS transistor has four electrical terminals: source, drain, gate, and the back-gate, also known as the body terminal. As the study of electronics has advanced, there has been an effort on implementing a Power, Performance and Area (PPA) technique that focuses on reducing the physical area and power consumption of a design while maintaining or improving performance specifications such as speed or frequency of operation. As a result, there is increased emphasis on reducing the channel length of a MOS transistor, which helps in improving the PPA of the design. Another way of improving the PPA is by applying a voltage at the back-gate terminal of the transistor.
Threshold voltage is defined as the minimum amount of voltage difference needed between the gate and source terminal of a transistor that allows an electric current to flow through the drain-source channel. In other words, as the gate to source voltage becomes equal to or greater than the threshold voltage, the transistor is said to be turned on (enabled) and current starts flowing through the channel. When the gate to source voltage drops below the threshold voltage, the transistor turns off (disabled) and ideally no current can flow through the channel. However, there may be some leakage or stand-by current flowing even when the transistor is off and this may be considered as an undesirable condition in most cases. It is well-known that any change in the threshold voltage has an impact on the PPA of the transistor device. For example, if the threshold voltage becomes smaller, the transistor can operate with smaller gate to source voltage, which may help in reducing power consumption. Also, reducing the threshold voltage helps in increasing the drive strength and frequency of the operation of the transistor, which may result in a smaller transistor device. Alternatively, increasing the threshold voltage may help in reducing the leakage or stand-by current during inactive or power-down mode.
The value of the threshold voltage is dependent on several factors. One of the factors that can decrease or increase the value of the threshold voltage is the amount of voltage applied at the back-gate terminal. The type of action in which threshold voltage is reduced by application of back-bias voltage is called forward back-biasing (FBB). Conversely, the type of action that causes the threshold voltage to increase by the application of back-bias voltage is called reverse back-biasing (RBB). Transistors having a back-gate terminal include silicon-on-insulator (SOI), such as fully depleted SOI (FD-SOI) transistors, bulk planar transistors, FinFet transistors, nanosheet transistors, and vertical transistors, to name a few. FD-SOI transistors are well-suited for such applications due to the possibility of applying full range (both positive and negative) back-bias voltage. This is due to the fact that for FD-SOI transistor, the forward and reverse back-bias voltage maximum upper range is limited only by the breakdown voltage of the reverse biased parasitic substrate diodes. This range may reach around 2-4 or higher times the maximum rated supply voltage at which the transistor device can normally operate. This maximum upper limit is much higher than many other MOS transistor types, e.g., bulk planar or FinFet transistors as their upper range is restricted in order to avoid their reverse-biased parasitic (source or drain) diffusion layer diodes becoming forward-biased. As a result, the potential variation in threshold voltage in the FD-SOI transistor type can be much higher as compared to the other MOS transistor device types. While this variation in threshold voltage is desirable and in some extreme cases, it may result in zero or twice the threshold voltage value. However, most back-gate biasing applications may need a smaller back-gate biasing voltage range that may be well below the maximum allowed upper limit, e.g., a back-gate biasing range from about −1.8V to about 1.8V may be enough for most applications. There are various approaches currently existing that attempt to supply transistors with the required voltages and properly bias transistors having back-gates.